Document FR 2 905 997 discloses mechanical devices for transmitting a threshold torque, such a device comprising a first element and a second element mounted to rotate relative to the first element, and retained relative to the first element until the force imparted by the second element exceeds a given threshold. Above the threshold, the second element is released and may move relative to the first element.
In particular, the device shown in FIGS. 5 and 6 of the above-mentioned document includes a first element in the form of a rod having left-hand and right-hand threads, a first nut and a second nut engaged on the rod and connected thereto via respective left-hand and right-hand helical connections. Alternating friction disks constrained to rotate with respective ones of the nuts are pressed against each other by a bearing member, specifically a spring member exerting a force on a sleeve that presses the disks together. The sleeve constitutes the second element of the device.
When a force is exerted on the rod in order to move it relative to the sleeve, the pressure exerted on the disks prevents the nut from turning, thereby having the effect of retaining the rod, until the exerted force exceeds a threshold at which the disks begin to slide relative to one another, thereby allowing the nut to turn and thus the rod to penetrate.
The above-described device is complex to fabricate since it requires right-hand threads to be made at a pitch that is large enough to implement a reversible helical connection between the rod and the nut.
Document EP 0 550 261 discloses a mechanical device for transmitting a threshold force in accordance with the precharacterizing portion of claim 1. In that device, an annular element (21) is secured to the second series of disks, rotary transmission means being interposed between said annular element and the second element in order to cause the annular element to move along the axis until it absorbs the clearance between itself and the bearing member and comes into abutment thereagainst in order to lift it during relative rotation between the annular element and the second element. There is thus a certain amount of latency time and an associated annular stroke between the moment when the annular element begins to turn relative to the second element and the moment when the bearing member is lifted.